The prior art includes for example patent application DE 10 2015 208 165 A1. This document describes a method for performing a parking brake process in a motor vehicle with a service brake and a parking brake, wherein a hydraulic force component and a mechanical force component are overlaid to achieve a total clamping force for the parking brake process. It is provided here that the two force components are overlaid for each parking brake process.
The prior art also includes patent application DE 10 2009 047 127 A1. This document describes a method for operating a parking brake of a vehicle, in particular a motor vehicle, which works in superposition mode, wherein the brake force of the parking brake can be applied by means of two different force-generating actuators which support each other in superposition mode. It is provided that the force-generating, in particular pressure-generating, actuator for support is already activated before the superposition.
Methods of this type which first build up hydraulic pressure and then apply the brake electromechanically have the problem that the hydraulic pressure is also effective on the vehicle axle with the applied parking brakes, usually the rear axle. Admittedly, a pressure sensor is present in the system, but this only measures the pressure at the brake master cylinder. It cannot ensure that the brake pressure is then also actually effective on the rear axle.